1. Technical Field
The invention relates in general to the adjustment of reception characteristics in an apparatus receiving radio-frequency transmissions and in particular to the automatic adjustment of timing and sampling frequency in an apparatus that receives OFDM modulated transmission.
2. Discussion of Related Art
Orthogonal frequency division multiplex (OFDM) refers to a modulation method where the transmitting device divides and attaches the transmitted signal to several subcarriers which are located on the frequency axis at regular intervals on a certain frequency band and which are sent simultaneously. Known radio-frequency communication systems that employ OFDM modulation include the DAB (Digital Audio Broadcasting) and DVB (Digital Video Broadcasting) systems. The former is specified in general outline in the ETS 300 401 standard by the European Broadcasting Union (EBU) and the European Telecommunications Standards Institute (ETSI), and the latter is specified in general outline in the prETS 300 800 draft standard by the same organizations. In these systems, a section of a digital signal to be transmitted on a certain subcarrier is encoded into phase and/or amplitude changes with respect to a certain known phase. That time slice of the transmitted signal during which the modulating phase state is constant separately at each subcarrier frequency is called a OFDM symbol, or a symbol in short.
In order for the receiving device to be able to correctly interpret the phase changes on the different subcarriers, the transmitter must include a certain phase reference in the signal. In the DAB system, the transmitted signal is divided into 24-ms or 96-ms frames, depending on the transmission mode, and each frame has a phase reference symbol at the beginning (after the null symbol) which indicates the phase reference simultaneously to all subcarriers. In the DVB system, the phase reference is included in the so-called pilot channels which are found in each symbol at intervals of twelve subcarriers.
Successful OFDM reception requires that the receiver maintains the correct symbol synchronization and sampling frequency. Symbol synchronization means that the receiver knows at which point of time each symbol begins and times the symbol detection correspondingly. Sampling frequency refers here to the frequency at which the A/D converter in the receiver takes samples from the received analog oscillation in order to convert the signal into digital form, whereby the A/D converter and subsequent circuits can interpret to which bits or bit combinations in the digital data flow the signal phase changes refer. In addition, the receiver has to maintain frequency synchronization, i.e. to tune the reception and mixing circuits so that the detected frequency band covers all subcarriers of the OFDM signal at an accuracy which is less than half of the difference between two adjacent subcarriers. Maintaining the symbol synchronization, sampling frequency and frequency synchronization is especially difficult if the transmitter and receiver are moving with respect to each other. The receiver may be located in a car, for example, and as the car moves around in an urban environment, the propagation path of the radio signal changes constantly, resulting in attenuation and reflections. The receiver may also be located in a satellite, and as the satellite moves, the speed difference between the receiver and the satellite changes, being possibly up to several kilometers per second.
A method is known from Finnish Patent Applications No. 962138 "Vastaanottimen tahdistuminen joutotilassa" and No. 962139 "Signaalin haku eraassa satelliittipuhe-linjarjestelmassa" to produce and maintain symbol synchronization and frequency synchronization in a radio system that does not use OFDM modulation. The method is based on the fact that a received signal includes on a certain control channel a synchronization sequence comprising bits in succession and a frequency information part which contains a short duration of pure sine wave at a desired frequency. The synchronization sequence belongs as part of the rest of the signal to a higher-power burst, and the receiver gets the coarse frame synchronization just by monitoring the highest received power peaks. In the finer synchronization, the receiver calculates how the various timing errors affect the detection of the synchronization sequence and deduces how the timing of the sampling should be corrected to make the received synchronization sequence match better with the known format of the synchronization sequence. Frequency fine-tuning is performed by calculating for a discrete Fourier transform from the received frequency information part and by tuning the reception and mixing frequencies so that the peak of the frequency error spectrum yielded by the Fourier transform is as close to zero as possible. In addition, the receiver monitors how the timing and frequency parameters change and predicts from them the required corrections while in idle state, i.e. receiving only occasionally.
The prior art method described above is not suitable to be used as the synchronization method for an OFDM receiver since an OFDM modulated signal does not include separate synchronization sequences or frequency information parts like the control channel signal of the I-CO Global Communications system described in said patent applications. There exists no efficient prior art method for maintaining the symbol synchronization, sampling frequency and frequency synchronization in an OFDM receiver.